myView.swift

import SwiftUI

func > (_ lhs: CGSize, _ rhs: CGSize) -> Bool {
    return lhs.width > rhs.width && lhs.height > rhs.height
}

struct MyView: View {
    
    @State var biggestSize: CGSize = .zero
    
    var h: CGFloat? {
        self.biggestSize == .zero ? nil : biggestSize.height
    }
    
    var body: some View {
        GeometryReader { geo in
            VStack {
            Text("ABC")
                .background(Color.green)
                .transformAnchorPreference(key: ViewFrameKey.self, value: .bounds) { (array, anchor) in
                    // Array is of type `ViewFrameKey.Value`. anchor is an instance of `Anchor<CGRect>`
                    // Here we grab the frame data for this view (specified by the anchor)
                    // from the GeometryProxy and add store it in our `ViewFrameKey` store.
                    array.append(.init(id: "firstRectangle", frame: geo[anchor]))
                }
                .onPreferenceChange(ViewFrameKey.self) { (frames) in
                    // frames is of type `ViewFrameKey.Value`
                    // Here we have a list of frames where `frames.first` is the last updated one.
                    guard let frame = frames.first else { return }
                    let size = frame.frame.size
                    if size > self.biggestSize {
                        self.biggestSize = size
                    }
                }
            .frame(height: self.h)
            
            Rectangle()
                .foregroundColor(.red)
                .transformAnchorPreference(key: ViewFrameKey.self, value: .bounds) { (array, anchor) in
                    // Array is of type `ViewFrameKey.Value`. anchor is an instance of `Anchor<CGRect>`
                    // Here we grab the frame data for this view (specified by the anchor)
                    // from the GeometryProxy and add store it in our `ViewFrameKey` store.
                    array.append(.init(id: "secondRectangle", frame: geo[anchor]))
                }
                .onPreferenceChange(ViewFrameKey.self) { (frames) in
                    // frames is of type `ViewFrameKey.Value`
                    // Here we have a list of frames where `frames.first` is the last updated one.
                    guard let frame = frames.first else { return }
                    let size = frame.frame.size
                    if size > self.biggestSize {
                        self.biggestSize = size
                    }
                }
                .frame(height: self.h)
        }
        }
    }
}

struct ViewFrame: Equatable {
    let id: String
    let frame: CGRect
    
    static func == (lhs: ViewFrame, rhs: ViewFrame) -> Bool {
        lhs.id == rhs.id && lhs.frame == rhs.frame
    }
}

struct ViewFrameKey: PreferenceKey {
    typealias Value = [ViewFrame]
    static var defaultValue: [ViewFrame] = []
    static func reduce(value: inout [ViewFrame], nextValue: () -> [ViewFrame]) {
        value.append(contentsOf: nextValue())
    }
}


struct MyView_Previews: PreviewProvider {
    static var previews: some View {
        MyView()
    }
}